Extrusion method for making polycarbonate

ABSTRACT

Extrusion of a mixture of an ester-substituted diaryl carbonate, such as bis-methyl salicyl carbonate, a dihydroxy aromatic compound such as bisphenol A and a transesterification catalyst such as tetrabutyl phosphonium acetate (TBPA) affords polycarbonate having a weight average molecular weight of greater than 20,000 Daltons. The extruder is equipped with one or more vacuum vents to remove by-product ester-substituted phenol. Similarly, a precursor polycarbonate having ester-substituted phenoxy endgroups, for example methyl salicyl endgroups, when subjected to extrusion affords a polycarbonate having a significantly increased molecular weight relative to the precursor polycarbonate. The reaction to form a higher molecular weight polycarbonate may be catalyzed by residual transesterification catalyst present in the precursor polycarbonate, or by a combination of any residual catalyst and an additional catalyst such as TBPA introduced in the extrusion step. Fries rearrangement products are not observed in the product polycarbonates.

BACKGROUND OF THE INVENTION

This invention relates to a method of preparing polycarbonates using anextruder to convert component monomers, ester substituted diarylcarbonates and dihydroxyaromatic compounds, into product polycarbonates.The invention further relates to the preparation of polycarbonates inwhich a precursor polycarbonate comprising ester-substituted phenoxyendgroups is subjected to extrusion to produce a polycarbonate having ahigher molecular weight. More particularly, the instant inventionrelates to the formation under mild conditions of polycarbonates havingextremely low levels of Fries rearrangement products and possessing ahigh level of endcapping.

Polycarbonates, such as bisphenol A polycarbonate, are typicallyprepared either by interfacial or melt polymerization methods. Thereaction of a bisphenol such as bisphenol A (BPA) with phosgene in thepresence of water, a solvent such as methylene chloride, an acidacceptor such as sodium hydroxide and a phase transfer catalyst such astriethylamine is typical of the interfacial methodology. The reaction ofbisphenol A with a source of carbonate units such as diphenyl carbonateat high temperature in the presence of a catalyst such as sodiumhydroxide is typical of currently employed melt polymerization methods.Each method is practiced on a large scale commercially and each presentssignificant drawbacks.

The interfacial method for making polycarbonate has several inherentdisadvantages. First it is a disadvantage to operate a process whichrequires phosgene as a reactant due to obvious safety concerns. Secondit is a disadvantage to operate a process which requires using largeamounts of an organic solvent because expensive precautions must betaken to guard against any adverse environmental impact. Third, theinterfacial method requires a relatively large amount of equipment andcapital investment. Fourth, the polycarbonate produced by theinterfacial process is prone to having inconsistent color, higher levelsof particulates, and higher chloride content, which can cause corrosion.

The melt method, although obviating the need for phosgene or a solventsuch as methylene chloride requires high temperatures and relativelylong reaction times. As a result, by-products may be formed at hightemperature, such as the products arising by Fries rearrangement ofcarbonate units along the growing polymer chains. Fries rearrangementgives rise to undesired and uncontrolled polymer branching which maynegatively impact the polymer's flow properties and performance. Themelt method further requires the use of complex processing equipmentcapable of operation at high temperature and low pressure, and capableof efficient agitation of the highly viscous polymer melt during therelatively long reaction times required to achieve high molecularweight.

Some years ago, it was reported in U.S. Pat. No. 4,323,668 thatpolycarbonate could be formed under relatively mild conditions byreacting a bisphenol such as BPA with the diaryl carbonate formed byreaction phosgene with methyl salicylate. The method used relativelyhigh levels of transesterification catalysts such as lithium stearate inorder to achieve high molecular weight polycarbonate. High catalystloadings are particularly undesirable in melt polycarbonate reactionssince the catalyst remains in the product polycarbonate following thereaction. The presence of a transesterification catalyst in thepolycarbonate may shorten the useful life span of articles madetherefrom by promoting increased water absorption, polymer degradationat high temperatures and discoloration.

It would be desirable, therefore, to minimize the amount of catalystrequired in the melt preparation of polycarbonate from bisphenols andester substituted diaryl carbonates such as bis-methyl salicyl carbonate(BMSC). In addition, it would be desirable to provide a method for themelt preparation of polycarbonate using simple melt mixing equipmentsuch as an extruder.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method for the preparation ofpolycarbonate comprising extruding at one or more temperatures in atemperature range and at one or more screw speeds in a screw speedrange, at least one starting material selected from the group consistingof

(A) a mixture comprising an ester-substituted diaryl carbonate, atransesterification catalyst and at least one dihydroxy aromaticcompound; and

(B) at least one precursor polycarbonate comprising ester-substitutedphenoxy terminal groups.

The present invention further relates to a single step method forpreparing highly endcapped, polycarbonates having very low levels ofFries rearrangement products, said polycarbonates comprising estersubstituted phenoxy endgroups.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be understood more readily by reference to thefollowing detailed description of preferred embodiments of the inventionand the examples included therein. In the following specification andthe claims which follow, reference will be made to a number of termswhich shall be defined to have the following meanings:

The singular forms “a”, “an” and “the” include plural referents unlessthe context clearly dictates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where the event occurs and instances where it does not.

As used herein the term “polycarbonate” refers to polycarbonatesincorporating structural units derived from one or more dihydroxyaromatic compounds and includes copolycarbonates and polyestercarbonates.

As used herein, the term “melt polycarbonate” refers to a polycarbonatemade by the transesterification of a diaryl carbonate with a dihydroxyaromatic compound.

As used herein the term “precursor polycarbonate” refers to apolycarbonate which when subjected to extrusion in the presence of atransesterification catalyst affords a polycarbonate having a highermolecular weight after the extrusion than before it.

“BPA” is herein defined as bisphenol A or2,2-bis(4-hydroxyphenyl)propane.

“Catalyst system” as used herein refers to the catalyst or catalyststhat catalyze the transesterification of the bisphenol with the diarylcarbonate in the melt process.

The terms “bisphenol”, “diphenol” and “dihydric phenol” as used hereinare synonymous.

“Catalytically effective amount” refers to the amount of the catalyst atwhich catalytic performance is exhibited.

As used herein the term “Fries product” is defined as a structural unitof the product polycarbonate which upon hydrolysis of the productpolycarbonate affords a carboxy-substituted dihydroxy aromatic compoundbearing a carboxy group adjacent to one or both of the hydroxy groups ofsaid carboxy-substituted dihydroxy aromatic compound. For example, inbisphenol A polycarbonate prepared by a melt reaction method in whichFries reaction occurs, the Fries product comprises structure VIII below,which affords 2-carboxy bisphenol A upon complete hydrolysis of theproduct polycarbonate.

The terms “Fries product” and “Fries group” are used interchangeablyherein.

The terms “Fries reaction” and “Fries rearrangement” are usedinterchangeably herein.

The terms “double screw extruder” and “twin screw extruder” are usedinterchangeably herein.

As used herein the term “monofunctional phenol” means a phenolcomprising a single reactive hydroxy group.

As used herein the term “aliphatic radical” refers to a radical having avalence of at least one comprising a linear or branched array of atomswhich is not cyclic. The array may include heteroatoms such as nitrogen,sulfur and oxygen or may be composed exclusively of carbon and hydrogen.Examples of aliphatic radicals include methyl, methylene, ethyl,ethylene, hexyl, hexamethylene and the like.

As used herein the term “aromatic radical” refers to a radical having avalence of at least one comprising at least one aromatic group. Examplesof aromatic radicals include phenyl, pyridyl, furanyl, thienyl,naphthyl, phenylene, and biphenyl. The term includes groups containingboth aromatic and aliphatic components, for example a benzyl group.

As used herein the term “cycloaliphatic radical” refers to a radicalhaving a valance of at least one comprising an array of atoms which iscyclic but which is not aromatic. The array may include heteroatoms suchas nitrogen, sulfur and oxygen or may be composed exclusively of carbonand hydrogen. Examples of cycloaliphatic radicals include cyclcopropyl,cyclopentyl cyclohexyl, tetrahydrofuranyl and the like.

According to the method of the present invention, extruding a startingmaterial (A), a mixture comprising an ester-substituted diarylcarbonate, a transesterification catalyst and at least one dihydroxyaromatic compound; or starting material (B), at least one precursorpolycarbonate comprising ester-substituted phenoxy terminal groups;affords a product polycarbonate. In some instances the method accordingto the present invention employs both starting materials (A) and (B), aswhere an ester-substituted diaryl carbonate, a dihydroxy aromaticcompound and a transesterification catalyst are first partially reactedto form a mixture comprising said ester-substituted diaryl carbonate, adihydroxy aromatic compound and a transesterification catalyst as well aprecursor polycarbonate comprising ester-substituted phenoxy endgroups,and said mixture is then extruded.

In one aspect the of the present invention the product polycarbonate isprepared by introducing an ester substituted diaryl carbonate, at leastone dihydroxy aromatic compound, and a transesterification catalyst intoan extruder to form a molten mixture in which reaction between carbonategroups and hydroxyl groups occurs giving rise to polycarbonate productand ester-substituted phenol by-product. The extruder may be equippedwith vacuum vents which serve to remove the ester-substituted phenolby-product and thus drive the polymerization reaction toward completion.The molecular weight of the polycarbonate product may be controlled bycontrolling, among other factors, the feed rate of the reactants, thetype of extruder, the extruder screw design and configuration, theresidence time in the extruder, the reaction temperature and the numberof vacuum vents present on the extruder and the pressure at which saidvacuum vents are operated. The molecular weight of the polycarbonateproduct may also depend upon the structures of the reactantester-substituted diaryl carbonate, dihydroxy aromatic compound, andtransesterification catalyst employed.

The ester-substituted diaryl carbonates according to the presentinvention include diaryl carbonates having structure I

wherein R¹ is independently at each occurrence C₁-C₂₀ alkyl radical, 2 iC₄-C₂₀ cycloalkyl radical or C₄-C₂₀ aromatic radical, R is independentlyat each occurrence a halogen atom, cyano group, nitro group, C₁-C₂₀alkyl radical, C₄-C₂₀ cycloalkyl radical, C₄-C₂₀ aromatic radical,C₁-C₂₀ alkoxy radical, C₄-C₂₀ cycloalkoxy radical, C₄-C₂₀ aryloxyradical, C₁-C₂₀ alkylthio radical, C₄-C₂₀ cycloalkylthio radical, C₄-C₂₀arylthio radical, C₁-C₂₀ alkylsulfinyl radical, C₄-C₂₀cycloalkylsulfinyl radical, C₄-C₂₀ arylsulfinyl radical, C₁-C₂₀alkylsulfonyl radical, C₄-C₂₀ cycloalkylsulfonyl radical, C₄-C₂₀arylsulfonyl radical, C₁-C₂₀ alkoxycarbonyl radical, C₄-C₂₀cycloalkoxycarbonyl radical, C₄-C₂₀ aryloxycarbonyl radical, C₂-C₆₀alkylamino radical, C₆-C₆₀ cycloalkylamino radical, C₅-C₆₀ arylaminoradical, C₁-C₄₀ alkylaminocarbonyl radical, C₄-C₄₀cycloalkylaminocarbonyl radical, C₄-C₄₀ arylaminocarbonyl radical, andC₁-C₂₀ acylamino radical; and b is independently at each occurrence aninteger 0-4.

Ester-substituted diaryl carbonates I are exemplified by bis-methylsalicyl carbonate (CAS Registry No. 82091-12-1), bis-ethyl salicylcarbonate, bis-propyl salicyl carbonate, bis-butyl salicyl carbonate,bis-benzyl salicyl carbonate, bis-methyl 4-chlorosalicyl carbonate andthe like. Typically bis-methyl salicyl carbonate is preferred.

The dihydroxy aromatic compounds according to the present inventioninclude bisphenols having structure II

wherein R³-R¹⁰ are independently a hydrogen atom, halogen atom, nitrogroup, cyano group, C₁-C₂₀ alkyl radical C₄-C₂₀ cycloalkyl radical, orC₆-C₂₀ aryl radical; W is a bond, an oxygen atom, a sulfur atom, a SO₂group, a C₁-C₂₀ aliphatic radical, a C₆-C₂₀ aromatic radical, a C₆-C₂₀cycloaliphatic radical or the group,

wherein R¹¹ and R¹² are independently a hydrogen atom, C₁-C₂₀ alkylradical, C₄-C₂₀ cycloalkyl radical, or C₄-C₂₀ aryl radical; or R¹¹ andR¹² together form a C₄-C₂₀ cycloaliphatic ring which is optionallysubstituted by one or more C₁-C₂₀ alkyl, C₆-C₂₀ aryl, C₅.C₂, aralkyl,C₅-C₂₀ cycloalkyl groups or a combination thereof.

Suitable bisphenols II are illustrated by2,2-bis(4-hydroxyphenyl)propane (bisphenol A);2,2-bis(3-chloro-4-hydroxyphenyl)propane;2,2-bis(3-bromo-4-hydroxyphenyl)propane;2,2-bis(4-hydroxy-3-methylphenyl)propane;2,2-bis(4-hydroxy-3-isopropylphenyl)propane;2,2-bis(3-t-butyl-4-hydroxyphenyl)propane;2,2-bis(3-phenyl-4-hydroxyphenyl)propane;2,2-bis(3,5-dichloro-4-hydroxyphenyl)propane;2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane;2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane;2,2-bis(3-chloro-4-hydroxy-5-methylphenyl)propane;2,2-bis(3-bromo-4-hydroxy-5-methylphenyl)propane;2,2-bis(3-chloro-4-hydroxy-5-isopropylphenyl)propane;2,2-bis(3-bromo-4-hydroxy-5-isopropylphenyl)propane;2,2-bis(3-t-butyl-5-chloro-4-hydroxyphenyl)propane;2,2-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)propane;2,2-bis(3-chloro-5-phenyl-4-hydroxyphenyl)propane;2,2-bis(3-bromo-5-phenyl-4-hydroxyphenyl)propane;2,2-bis(3,5-disopropyl-4-hydroxyphenyl)propane;2,2-bis(3,5-di-t-butyl-4-hydroxyphenyl)propane;2,2-bis(3,5-diphenyl-4-hydroxyphenyl)propane;2,2-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl)propane;2,2-bis(4-hydroxy-2,3,5 ,6-tetrabromophenyl)propane;2,2-bis(4-hydroxy-2,3,5,6-tetramethylphenyl)propane;2,2-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)propane;2,2-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)propane;1,1-bis(4-hydroxyphenyl)cyclohexane;1,1-bis(3-chloro-4-hydroxyphenyl)cyclohexane;1,1-bis(3-bromo-4-hydroxyphenyl)cyclohexane;1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane;1,1-bis(4-hydroxy-3-isopropylphenyl)cyclohexane;1,1-bis(3-t-butyl-4-hydroxyphenyl)cyclohexane;1,1-bis(3-phenyl-4-hydroxyphenyl)cyclohexane;1,1-bis(3,5-dichloro-4-hydroxyphenyl)cyclohexane;1,1-bis(3,5-dibromo-4-hydroxyphenyl)cyclohexane;1,1-bis(3,5-dimethyl-4-hydroxyphenyl)cyclohexane;1,1-bis(3-chloro-4-hydroxy-5-methylphenyl)cyclohexane;1,1-bis(3-bromo-4-hydroxy-5-methylphenyl)cyclohexane;1,1-bis(3-chloro-4-hydroxy-5-isopropylphenyl)cyclohexane;1,1-bis(3-bromo-4-hydroxy-5-isopropylphenyl)cyclohexane;1,1-bis(3-t-butyl-5-chloro-4-hydroxyphenyl)cyclohexane;1,1-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)cyclohexane;1,1-bis(3-chloro-5-phenyl-4-hydroxyphenyl)cyclohexane;1,1-bis(3-bromo-5-phenyl-4-hydroxyphenyl)cyclohexane;1,1-bis(3,5-disopropyl-4-hydroxyphenyl)cyclohexane;1,1-bis(3,5-di-t-butyl-4-hydroxyphenyl)cyclohexane;1,1-bis(3,5-diphenyl-4-hydroxyphenyl)cyclohexane;1,1-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl)cyclohexane;1,1-bis(4-hydroxy-2,3,5,6-tetrabromophenyl)cyclohexane;1,1-bis(4-hydroxy-2,3,5,6-tetramethylphenyl )cyclohexane;1,1-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)cyclohexane;1,1-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)cyclohexane;1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3-chloro-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3-bromo-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(4-hydroxy-3-methylphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(4-hydroxy-3-isopropylphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3,5-dichloro-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3,5-dibromo-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3-chloro-4-hydroxy-5-methylphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3-bromo-4-hydroxy-5-methylphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3-chloro-4-hydroxy-5-isopropylphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3-bromo-4-hydroxy-5-isopropylphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3-t-butyl-5-chloro-4-hydroxy phenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;bis(3-chloro-5-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3-bromo-5-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3,5-disopropyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3,5-di-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(3,5-diphenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl)-3,3,5-trimethylcyclohexane;1,1-bis(4-hydroxy-2,3,5,6-tetrabromophenyl)-3,3,5-trimethylcyclohexane;1,1-bis(4-hydroxy-2,3,5,6-tetramethylphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;4,4-dihydroxy-1,1-biphenyl; 4,4′-dihydroxy-3, 3′-dimethyl-1,1-biphenyl;4,4′-dihydroxy-3,3′-dioctyl-1,1-biphenyl; 4,4′-dihydroxydiphenylether;4,4′-dihydroxydiphenylthioether;1,3-bis(2-(4-hydroxyphenyl)-2-propyl)benzene;1,3-bis(2-(4-hydroxy-3-methylphenyl)-2-propyl)benzene;1,4-bis(2-(4-hydroxyphenyl)-2-propyl)benzene and1,4-bis(2-(4-hydroxy-3-methylphenyl)-2-propyl)benzene. Bisphenol A ispreferred.

The polycarbonate prepared according to the method of the presentinvention comprises ester substituted phenoxy endgroups having structureIII

wherein R¹ and R² are defined as in structure I and b is an integer 0-4;or endgroups derived from structure III, for example, endgroupsintroduced by displacement of an ester substituted phenoxy endgrouphaving structure III by a monofunctional phenol such as p-cumylphenol.In one embodiment of the present invention structure III is the methylsalicyl group IV. The methyl salicyl endgroup IV

is preferred.

The present invention a provides a method for the preparation ofpolycarbonate, said method comprising extruding at least one startingmaterial selected from the group consisting of: (A) a mixture comprisingan ester-substituted diaryl carbonate, a transesterification catalystand at least one dihydroxy aromatic compound; and (B) at least oneprecursor polycarbonate comprising ester-substituted phenoxy endgroups.The extruder is operated at one or more temperatures in a temperaturerange, at least one of said temperatures being sufficient to promotereaction between hydroxy and carbonate groups present in the startingmaterial, thereby effecting polymer chain growth. The method of thepresent invention provides the product polycarbonate as an extrudate.The reaction between hydroxy and carbonate groups is advantageouslycatalyzed by a transesterification catalyst. Where starting material (A)is employed the transesterification catalyst is introduced into theextruder along with the ester-substituted diaryl carbonate and at leastone dihydroxy aromatic compound. Where starting material (B) isemployed, a transesterification catalyst may be added in addition to theprecursor polycarbonate being introduced into the extruder. Frequently,however, the precursor polycarbonate is itself prepared via a meltreaction between an ester-substituted diaryl carbonate and at least onedihydroxy aromatic compound in the presence of a transesterificationcatalyst. Precursor polycarbonates incorporating ester-substitutedphenoxy endgroups may be conveniently prepared by heating a mixture ofat least one dihydroxy aromatic compound, such as bisphenol A, with anester-substituted diaryl carbonate, such as bis-methyl salicylcarbonate, in the presence of transesterification catalyst, such astetrabutyl phosphonium acetate, at a temperature in a range between 150°C. and 200° C. and a pressure between about 1 mmHg and about 100 mmHgwhile removing by-product ester-substituted phenol, saidtransesterification catalyst being used in an amount corresponding tobetween about 1×10⁻⁸ and 1×10⁻³ moles catalyst per mole dihydroxyaromatic compound. The transesterification catalysts suitable for use inthe melt preparation of precursor polycarbonates comprising estersubstituted endgroups include those catalysts described herein. Suchtransesterification catalysts are reasonably stable under the conditionsof the melt preparation of the precursor polycarbonates. Thus, theprecursor polycarbonate may contain sufficient residual-transesterification catalyst such that additional transesterificationcatalyst is often unnecessary to effect substantial molecular weightincrease upon extrusion of the precursor polycarbonate. The precursorpolycarbonate, starting material (B), may be introduced into theextruder in a variety of forms according to the method of the presentinvention, including as an amorphous powder, as a partially crystallinepowder and as a melt.

The amount of transesterification catalyst present according to themethod of the present invention is in a range between about 1×10⁻⁸ andabout 1×10⁻³, preferably between about 1×10⁻⁷ and about 1×10⁻³, andstill more preferably between about 1×10⁻⁶ and about 5×10⁻⁴ molescatalyst per mole dihydroxy aromatic compound employed in the case ofstarting material (A), or in the case of starting material (B) per moleof structural units present in the precursor polycarbonate which arederived from a dihydroxy aromatic compound. The amount oftransesterification catalyst present in catalyst systems having multiplecomponents, for example sodium hydroxide and tetrabutyl phosphoniumacetate, is expressed as the sum of the number of moles of eachcomponent of the catalyst system per mole dihydroxy aromatic compound inthe case of starting material (A), or in the case of starting material(B) per mole of structural units present in the precursor polycarbonatewhich are derived from a dihydroxy aromatic compound.

In one embodiment of the present invention a precursor polycarbonatecomprising repeat units V

is employed as starting material (B), wherein said precursorpolycarbonate comprises residual transesterification catalyst, saidcatalyst being present in an amount such that the mole ratio oftransesterification catalyst to bisphenol A-derived structural units Vis in a range between about 1×10⁻⁸ and about 1×10⁻³, preferably betweenabout 1×10⁻⁷ and about 1×10⁻³ and still more preferably between about1×10⁻⁶ and about 5×10⁻⁴.

Suitable transesterification catalysts according to the method of thepresent invention include salts of alkaline earth metals, salts ofalkali metals, quaternary ammonium compounds, quaternary phosphoniumions, and mixtures thereof. Suitable transesterification catalystsinclude quaternary ammonium compounds comprising structure VI

wherein R¹-R¹⁶ are independently a C₁-C₂₀ alkyl radical, C₄-C₂₀cycloalkyl radical or a C₄-C₂₀ aryl radical and X⁻ is an organic orinorganic anion. Anions X⁻ include hydroxide, halide, carboxylate,phenoxide, sulfonate, sulfate, carbonate, and bicarbonate. In oneembodiment of the present invention the transesterification catalystcomprises tetramethyl ammonium hydroxide.

Suitable transesterification catalysts include quaternary phosphoniumcompounds comprising structure VII

wherein R¹⁷-R²⁰ and X⁻ are defined as in structure VI. In one embodimentof the present invention the transesterification catalyst comprisestetrabutyl phosphonium acetate.

Where X⁻ is a polyvalent anion such as carbonate or sulfate it isunderstood that the positive and negative charges in structures VI andVII are properly balanced. For example, where R¹⁷-R²⁰ in structure VIIare each methyl groups and X⁻ is carbonate, it is understood that X⁻represents ½ (CO₃ ⁻²).

In one embodiment of the present invention the transesterificationcatalyst further comprises at least one alkali metal hydroxide, alkalineearth hydroxide or mixture thereof, in addition to a quaternary ammoniumcompound such as VI, a quaternary phosphonium compound such as VII, or amixture thereof. Sodium hydroxide in combination with tetrabutylphosphonium acetate illustrates such mixed catalyst systems. In catalystsystems comprising quaternary “onium” compounds such as VI or VIItogether with a metal hydroxide such as sodium hydroxide, it isfrequently preferred that the amount of “onium” compound be present inexcess relative to the metal hydroxide, preferably in an amountcorresponding to from about 10 to about 250 times the amount of metalhydroxide employed.

In one embodiment of the present invention the transesterificationcatalyst comprises at least one alkali metal salt of a carboxylic acid,an alkaline earth metal salt of a carboxylic acid or a mixture thereof.Salts of ethylene diamine tetracarboxylic acid (EDTA) have been found tobe particularly effective, among them Na₂Mg EDTA.

In yet another embodiment of the present invention thetransesterification catalyst comprises the salt of a non-volatileinorganic acid. By “nonvolatile” it is meant that the referencedcompounds have no appreciable vapor pressure at ambient temperature andpressure. In particular, these compounds are not volatile attemperatures at which melt polymerizations of polycarbonate aretypically conducted. The salts of nonvolatile acids according thepresent invention are alkali metal salts of phosphites; alkaline earthmetal salts of phosphites; alkali metal salts of phosphates; andalkaline earth metal salts of phosphates. Suitable salts of nonvolatileacids include NaH₂PO₃, NaH₂PO₄, Na₂H₂PO₃, KH₂PO₄, CsH₂PO₄, Cs₂H₂PO₄, anda mixture thereof In one embodiment, the salt of the nonvolatile acid isCsH₂PO₄. In one embodiment of the present invention thetransesterification catalyst comprises both the salt of a non-volatileacid and a basic co-catalyst such as an alkali metal hydroxide. Thisconcept is exemplified by the use of a combination of NaH₂PO₄ and sodiumhydroxide as the transesterification catalyst.

In one embodiment of the present invention, the starting material (A)comprises between about 0.9 and about 1.25, preferably about 0.95 toabout 1.05 moles of ester-substituted diaryl carbonate per mole ofaromatic dihydroxy compound present in the mixture, and between about1.0×10⁻⁸ to about 1×10⁻³, preferably between about 1.0×10⁻⁶ to about5×10⁻⁴ moles of transesterification catalyst per mole of aromaticdihydroxy compound present in the mixture.

The components of starting material (A); ester-substituted diarylcarbonate, at least one dihydroxy aromatic compound, atransesterification catalyst, and optionally a monofunctional phenol maybe introduced into the extruder through the same or separate feed inletsand the rates of introduction of said components and said optionalmonofunctional phenol may be varied to control the molar ratios of thereactants and in this manner to control the physical properties of theproduct polycarbonate such as molecular weight and endgroup identity.The method of the present invention thus allows for adjustment of theproduct polycarbonate molecular weight within the context of acontinuous process. For example, a slight adjustment in the relativerates of introduction of ester-substituted diaryl carbonate, dihydroxyaromatic compound and optionally monofunctional phenol may be madeduring a continuous extrusion operation to vary slightly the molecularweight of the product polycarbonate. Conversely, substantial changes inthe product polycarbonate molecular weight may be made, as in forinstance a polymer grade change, by more substantial adjustment in therelative rates of introduction of ester-substituted diaryl carbonate,dihydroxy aromatic compound and optionally monofunctional phenol.

The extruder employed according to the method of the present inventionis operated at one or more temperatures in a range between about 100° C.and about 350° C., preferably between about 250° C. and about 300° C.

The extruder, which may be a single screw or multiple screw extruder isoperated at one or more screw speeds in a screw speed range, said rangebeing between about 50 revolutions per minute (rpm) and about 500 rpm,preferably between about 200 rpm and about 400 rpm.

As mentioned, the extruder may be equipped with a vacuum vent. A vacuumvent is necessary in instances where a large amount of ester-substitutedphenol by-product is evolved during the extrusion, as in the casewherein starting material (A) is employed. In instances wherein thetotal amount of by-product ester-substituted phenol is relatively small,as in the case of the extrusion of a precursor polycarbonate comprisingester-substituted phenoxy endgroups IV, said precursor polycarbonatehaving substantial molecular weight, for example a weight averagemolecular weight of at least about 16000 Daltons relative to apolycarbonate standard, the use of vacuum vents is optional. In general,however, it is found expedient to practice the method of the presentinvention on an extruder comprising at least one vacuum vent. Frequentlyit is preferred to have two or more vacuum vents. In some embodiments ofthe present invention 4 vacuum vents are employed. More generally, themethod of the present invention utilizes an extruder equipped with asufficient number of vacuum vents to convert the starting material topolycarbonate having the desired molecular weight. The vacuum vents areoperated at reduced pressure, usually in a range between about 1 andabout 700 mmHg, preferably between about 10 and about 50 mmHg.

Extruders which may be employed according to the method of the presentinvention include co-rotating, intermeshing double screw extruders;counter-rotating, non-intermeshing double screw extruders; single screwreciprocating extruders, and single screw non-reciprocating extruders.

In one embodiment of the present invention the mixture introduced intothe extruder further comprises a chainstopper. The chainstopper may beincluded with starting material (A) or starting material (B) or amixture thereof, and can be used to limit the molecular weight of theproduct polymer or alter its physical properties such as glasstransition temperature or static charge carrying properties. Suitablechainstoppers include monofunctional phenols, for example p-cumylphenol;2,6-xylenol; 4-t-butylphenol; p-cresol; 1-naphthol; 2-naphthol;cardanol; 3,5-di-t-butylphenol, p-nonylphenol; p-octadecylphenol; andphenol. In alternative eembodiments of the present invention themonofunctional phenol may be added at an intermediate stage of thepolymerization or after its completion, as where the monofunctionalphenol is added downstream of the feed inlet used to introduce startingmaterials (A) or (B). In such alternative embodiments the chainstoppermay exert a controlling effect upon the molecular weight of the productpolycarbonate and will control the identity of the polymer terminalgroups.

The method of the present invention provides a product polycarbonatehaving a weight average molecular weight, as determined by gelpermeation chromatography, in a range between about 10,000 and about100,000 Daltons, preferably between about 15,000 and about 60,000Daltons, and still more preferably between about 15,000 and about 50,000Daltons; said product polycarbonate having less than about 1000,preferably less than about 500, and still more preferably less thanabout 100 parts per million (ppm) Fries product. Structure VIH belowillustrates the Fries product structure present in a polycarbonateprepared from bisphenol A. As indicated, the Fries product may serve asa site for polymer branching, the wavy lines indicating polymer chainstructure.

Polycarbonates prepared using the method of the present invention may beblended with conventional additives such as heat stabilizers, moldrelease agents and UV stabilizers and molded into various moldedarticles such as optical disks, optical lenses, automobile lampcomponents and the like. Further, the polycarbonates prepared using themethod of the present invention may be blended with other polymericmaterials, for example, other polycarbonates, polyestercarbonates,polyesters and olefin polymers such as ABS.

EXAMPLES

The following examples are set forth to provide those of ordinary skillin the art with a detailed description of how the methods claimed hereinare evaluated, and are not intended to limit the scope of what theinventors regard as their invention. Unless indicated otherwise, partsare by weight, temperature is in ° C.

Molecular weights are reported as number average (M_(n)) or weightaverage (M_(w)) molecular weight and were determined by gel permeationchromatography (GPC) analysis, using a polycarbonate molecular weightstandard to construct a broad standard calibration curve against whichpolymer molecular weights were determined. The temperature of the gelpermeation columns was about 25° C. and the mobile phase was chloroform.

Fries content was measured by the KOH methanolysis of resin and isreported as parts per million (ppm). The Fries product content ofpolycarbonates was determined as follows. First, 0.50 grams ofpolycarbonate was dissolved in 4.0 ml of THF (containing p-terphenyl asinternal standard). Next, 3.0 ml of 18% KOH in methanol was added tothis solution. The resulting mixture was stirred for two hours at thistemperature. Next, 1.0 ml of acetic acid was added, and the mixture wasstirred for 5 minutes. Potassium acetate by-product was allowed tocrystallize over 1 hour. The solid was filtered off and the resultingfiltrate was analyzed by liquid chromatograph using p-terphenyl as theinternal standard. No Fries product was detected in Examples 6-9, Table3.

Examples 1-2 and Comparative Example 1

Introduction at a feed rate of between about 3 and about 6 pounds perhour of a mixture of bisphenol A, BMSC or diphenyl carbonate, andtetrabutyl phosphonium acetate catalyst (2.5×10⁻⁴ moles catalyst permole BPA) into the throat of a 20 mm twin screw extruder equipped with 4vacuum vents operated at about 25 mmHg, the extruder having zonetemperatures in a range between about 260° C. and about 300° C., theextruder being operated at a screw speed in a range between about 50 andabout 350 rpm, afforded polycarbonate having substantial molecularweight when BMSC was employed. The ratio of diaryl carbonate (DPC orBMSC) to BPA, “DAC/BPA”, is given in Table 1 and represents the molarratio of diaryl carbonate to BPA employed. “%EC” represents thepercentage of product polycarbonate endgroups which are not hydroxylgroups and “Tg° C” is the glass transition temperature in degreescentigrade. The designation “CE-1” means Comparative Example 1. Data areprovided for Examples 1 and 2 in Table 1 which illustrate an embodimentof the method of the present invention. In Examples 1 and 2polycarbonate having weight average molecular weights in excess of20,000 Daltons is obtained. Comparative Example 1 illustrates the resultobtained when the diaryl carbonate employed is diphenyl carbonate. Theuse of diphenyl carbonate affords only low molecular weight oligomericmaterial and substantial amounts of feed input is lost at the vacuumvents due to the persistence of unreacted diphenyl carbonate andbisphenol A.

TABLE 1 DIRECT POLYMERIZATION OF BISPHENOL A AND BMSC Example DAC/BPA MwMn % EC Tg° C. Example 1 1.017 21489 9410 96.3 147 Example 2 1.017 237899937 76.2 CE-1 1.08 2369 1797 115

In addition to the process illustrated by Examples 1 and 2, the presentinvention provides for the preparation of polycarbonate in a two stepprocess wherein an ester-substituted diaryl carbonate is first meltreacted with at least one dihydroxy aromatic compound to give aprecursor polycarbonate having a reduced molecular weight. The precursorpolycarbonate is then extruded to afford a substantially highermolecular weight polycarbonate. These aspects of the present inventionare illustrated by Examples 3-5.

The precursor polycarbonates of Examples 3-5 were prepared by the meltreaction of BMSC and BPA in the presence of the catalysts tetrabutylphosphonium acetate and disodium magnesium ethylenediamine tetraacetate.The melt polymerization reaction of Example 5 was carried out in thepresence of 5.07 mole percent p-cumylphenol. The melt polymerizationreactions were carried out as follows: Partially crystalline precursorpolycarbonates were made in a 4 L glass reaction vessel adapted fordistillation under reduced pressure. The reaction vessel was equippedwith a stainless steel agitator, a water cooled condenser and chilled,graduated receiving flask. Prior to its use the reaction vessel wasrinsed with concentrated sulfuric acid and then deionized water(18-Mohm) until the rinse was neutral. The reaction vessel was thendried over night in a drying oven. The reaction vessel was heated bymeans of an electric heating mantle equipped with PID temperaturecontrollers. The temperature of the mantle was measured at themantle-reaction vessel interface. The pressure inside the reactionvessel was controlled by means of a vacuum pump.

The reaction vessel was charged at ambient temperature and pressure withsolid bisphenol A (General Electric Plastics Japan Ltd., 4.976 mol) andsolid bis-methyl salicyl carbonate (5.064 mol) and optionallymonofunctional phenol endcapping agent. The catalysttetrabutylphosphonium acetate (2.5×10⁻⁴ mole/mole BPA) and co-catalystEDTA MgNa2 (1.0×0⁻⁶ mole/mole BPA) were added as aqueous solutions andthe reaction vessel was assembled. The reaction vessel was thenevacuated briefly and nitrogen was reintroduced. This step was repeatedthree times. The reaction vessel was then heated to 190° C. and thepressure lowered to less than about 10 mmHg. After 20 minutes thereactants were sufficiently melted to allow for stirring. Oncedistillation began, the temperature was reduced to 170° C. Theseconditions were maintained until about 25 percent of the targetdistillate had been collected. The pressure was then increased tobetween 30 and 40 mmHg. Within 30 minutes the contents of the reactionvessel turned white and once 30% of the theoretical amount distillatehad been collected the mantle heater was turned off and the pressureincreased to 500 mmHg. The oligomers were left to cool overnight beforebeing collected and ground for extrusion. Table 2 provides data forprecursor polycarbonates prepared by this method. No Fries rearrangementproducts could be detected in the precursor polycarbonates prepared inExamples 3-5. The column heading “%ESEG” indicates the percent of theprecursor polycarbonate endgroups which are “ester-substitutedendgroups” having structure III.

TABLE 2 MELT PREPARATION OF PRECURSOR POLYCARBOMNATES Example DAC/BPA MwMn % EC % ESEG Example 3 1.017 16566 7592 75 75% Example 4 1.017 167847519 75 75% Example 5 1.017 14022 6774 66 33%

The partially crystalline precursor polycarbonates prepared in Examples3-5 were divided into batches and ground to a powder in a Henschelmixer. Additional tetrabutyl phosphonium acetate (TBPA) catalyst, 150ppm based upon the total weight of the precursor polycarbonate, wasadded to one of the batches (See Example 9) during grinding in theHenschel mixer. Otherwise, the only catalyst present during theextrusion was the residual catalyst remaining in the precursorpolycarbonates from the melt polymerization reaction used to form theprecursor polycarbonates. In Comparative Example No. 2 the precursorpolycarbonate was amorphous and was prepared by the melt reaction ofdiphenyl carbonate with bisphenol A. The precursor polycarbonates werethen introduced at a feed rate of 3-6 pounds per hour into the throat ofa 20 mm twin screw extruder equipped with 2 to 4 vacuum vents operatedat about 25 mmHg and the extruder having zone temperatures between about260° C. and about 300° C. The extruder was operated at a screw speed ofabout 350 rpm. The polycarbonate emerging from the extruder was found topossess substantially increased molecular weight relative to theprecursor polycarbonate produced in the melt reaction. Data for theproduct polycarbonates are provided in Table 3.

TABLE 3 CONVERSION OF PRECURSOR POLYCARBONATES TO PRODUCT POLYCARBONATESVIA EXTRUSION Mw Mw Example TBPA precursor product Fries Level [OH] % ECExample 6 0 16566 26820 n.d. 157 94.6 Example 7 0 16566 32395 n.d.  4098.4 Example 8 0 16784 31561 n.d.  45 98.2 Example 9 150  16784 32980n.d.  38 98.4 Example 10 0 14022 16852 — 863 80.2 CE-2 0  6535 10467 —813 75.5

The data in Table 3 illustrate that the method of the present inventionmay be used to prepare significantly higher molecular weightpolycarbonates from precursor polycarbonates having lower molecularweight. The column heading “TBPA” refers to the amount of TBPA catalystin parts per million (ppm) added to the precursor polycarbonate. Thecolumn heading “Fries Level” indicates the concentration of Friesproduct in the product polycarbonate. The symbol “n.d.” means “notdetected” and indicates the absence of Fries product. The heading “[OH]”refers to the total concentration of OH groups in the productpolycarbonate. “%EC” represents the percentage of polymer chain ends notterminating in a hydroxyl group. Hydroxyl endgroup concentrations weredetermined by quantitative infrared spectroscopy. Monofunctional phenoland salicyl endgroup concentrations were determined by HPLC analysisafter product solvolysis. 64Examples 6-9 illustrate the substantialincrease in molecular weight which occurs as the precursorpolycarbonates incorporating ester-substituted phenoxy endgroups aresubjected to extrusion. Polycarbonates possessing a substantial level ofendcapping, %EC, but lacking ester-substituted phenoxy endgroups, forexample the polycarbonate of Comparative Example 2, show only modestmolecular weight enhancements upon extrusion. Additionally, precursorpolycarbonates incorporating a substantial amount non-ester-substitutedphenoxy endgroups, for example Example 10, show much more limitedmolecular weight enhancement upon extrusion than do precursorpolycarbonates in which essentially all of the non-hydroxy endgroups areester-substituted phenoxy groups. The data in Table 3 further illustratethat the method of the present invention allows the preparation ofpolycarbonate which is essentially free of Fries rearrangement product.Thus, the precursor polycarbonates can be prepared and subsequentlyextruded to afford high molecular weight polycarbonate without theformation of Fries rearrangement products.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood by thoseskilled in the art that variations and modifications can be effectedwithin the spirit and scope of the invention.

What is claimed is:
 1. A method for the preparation of polycarbonatecomprising extruding at one or more temperatures in a temperature rangebetween about 100° C. and about 350° C. and at one or more screw speedsin a screw speed range between about 50 and about 500 revolutions perminute, at least one starting material selected from the groupconsisting of (A) a mixture comprising an ester-substituted diarylcarbonate, a transesterification catalyst and at least one dihydroxyaromatic compound; and (B) at least one precursor polycarbonatecomprising ester-substituted phenoxy terminal groups.
 2. A methodaccording to claim 1 wherein said ester-substituted diaryl carbonate hasstructure I

wherein R¹ is independently at each occurrence C₁-C₂₀ alkyl radical,C₄-C₂₀ cycloalkyl radical or C₄-C₂₀ aromatic radical, R² isindependently at each occurrence a 15 halogen atom, cyano group, nitrogroup, C₁-C₂₀ alkyl radical, C₄-C₂₀ cycloalkyl radical, C₄-C₂₀ aromaticradical, C₁-C₂₀ alkoxy radical, C₄-C₂₀ cycloalkoxy radical, C₄-C₂₀aryloxy radical, C₁-C₂₀ alkylthio radical, C₄-C₂₀ cycloalkylthioradical, C₄-C₂₀ arylthio radical, C₁-C₂₀ alkylsulfinyl radical, C₄-C₂₀cycloalkylsulfinyl radical, C₄-C₂₀ arylsulfinyl radical, C₁-C₂₀alkylsulfonyl radical, C₄-C₂₀ cycloalkylsulfonyl radical, C₄-C₂₀arylsulfonyl radical, C₁-C₂₀ alkoxycarbonyl radical, C₄-C₂₀cycloalkoxycarbonyl radical, C₄-C₂₀ aryloxycarbonyl radical, C₂-C₆₀alkylamino radical, C₆-C₆₀ cycloalkylamino radical, C₅-C₆₀ arylaminoradical, C₁-C₄₀ alkylaminocarbonyl radical, C₄-C₄₀cycloalkylaminocarbonyl radical, C₄-C₄₀ arylaminocarbonyl radical, andC₁-C₂₀ acylamino radical; and b is independently at each occurrence aninteger 0-4.
 3. A method according to claim 2 wherein ester-substituteddiaryl carbonate is selected from the group comprising bis-methylsalicyl carbonate, bis-propyl salicyl carbonate, and bis-benzyl salicylcarbonate.
 4. A method according to claim 1 wherein said dihydroxyaromatic compound is a bisphenol having structure II

wherein R³-R¹⁰ are independently a hydrogen atom, halogen atom, nitrogroup, cyano group, C₁-C₂₀ alkyl radical C₄-C₂₀ cycloalkyl radical, orC₆-C₂₀ aryl radical; W is a bond, an oxygen atom, a sulfur atom, a SO₂group, a C₁-C₂₀ aliphatic radical, a C₆-C₂₀ aromatic radical, a C₆-C₂₀cycloaliphatic radical or the group

wherein R¹¹ and R¹² are independently a hydrogen atom, C₁-C₂₀ alkylradical, C₄-C₂₀ cycloalkyl radical, or C₄-C₂₀ aryl radical; or R¹¹ andR¹² together form a C₄-C₂₀ cycloaliphatic ring which is optionallysubstituted by one or more C₁-C₂₀ alkyl, C₆-C₂₀ aryl, C₅-C₂₁ aralkyl,C₅-C₂₀ cycloalkyl groups or a combination thereof.
 5. A method accordingto claim 4 wherein said bisphenol is bisphenol A.
 6. A method accordingto claim 1 wherein said precursor polycarbonate comprisesester-substituted phenoxy terminal groups having structure III

wherein R¹ is a C₁-C₂₀ alkyl radical, C₄-C₂₀ cycloalkyl radical orC₄-C₂₀ aromatic radical, R² is independently at each occurrence ahalogen atom, cyano group, nitro group, C₁-C₂₀ alkyl radical, C₄-C₂₀cycloalkyl radical, C₄-C₂₀ aromatic radical, C₁-C₂₀ alkoxy radical,C₄-C₂₀ cycloalkoxy radical, C₄-C₂₀ aryloxy radical, C₁-C₂₀ alkylthioradical, C₄-C₂₀ cycloalkylthio radical, C₄-C₂₀ arylthio radical, C₁-C₂₀alkylsulfinyl radical, C₄-C₂₀ cycloalkylsulfinyl radical, C₄-C₂₀arylsulfinyl radical, C₁-C₂₀ alkylsulfonyl radical, C₄-C₂₀cycloalkylsulfonyl radical, C₄-C₂₀ arylsulfonyl radical, C₁-C₂₀alkoxycarbonyl radical, C₄-C₂₀ cycloalkoxycarbonyl radical, C₄-C₂₀aryloxycarbonyl radical, C₂-C₆₀ alkylamino radical, C₆-C₆₀cycloalkylamino radical, C₅-C₆₀ arylamino radical, C₁-C₄₀alkylaminocarbonyl radical, C₄-C₄₀ cycloalkylaminocarbonyl radical,C₄-C₄₀ arylaminocarbonyl radical, and C₁-C₂₀ acylamino radical; and b isan integer 0-4.
 7. A method according to claim 6 wherein said precursorpolycarbonate comprises ester-substituted phenoxy terminal groups havingstructure IV


8. A method according to claim 7 wherein said precursor polycarbonate ispartially crystalline.
 9. A method according to claim 8 wherein saidprecursor polycarbonate has a crystallinity of between 10 and 40percent.
 10. A method according to claim 6 wherein said precursor 15polycarbonate comprises bisphenol A repeat units V


11. A method according to claim 1 wherein said transesterificationcatalyst comprises a quaternary ammonium compound, a quaternaryphosphonium compound or a mixture thereof.
 12. A method according toclaim 11 wherein said quaternary ammonium compound has structurecomprises structure VI

wherein R¹³-R¹⁶ are independently a C₁-C₂₀ alkyl radical, C₄-C₂₀cycloalkyl radical or a C₄-C₂₀ aryl radical and X⁻ is an organic orinorganic anion.
 13. A method according to claim 12 wherein said anionis selected from the group consisting of hydroxide, halide, carboxylate,phenoxide, sulfonate, sulfate, carbonate, and bicarbonate.
 14. A methodaccording to claim 12 wherein said quaternary ammonium compound istetramethyl ammonium hydroxide.
 15. A method according to claim 11wherein said phosphonium compound comprises structure VII

wherein R¹⁷-R²⁰ are independently a C₁-C₂₀ alkyl radical, C₄-C₂₀cycloalkyl radical or a C₄-C₂₀ aryl radical and X⁻ is an organic orinorganic anion.
 16. A method according to claim 15 wherein said anionis selected from the group consisting of hydroxide, halide, carboxylate,phenoxide sulfonate, sulfate, carbonate, and bicarbonate.
 17. A methodaccording to claim 15 wherein said quaternary phosphonium compound istetrabutyl phosphonium acetate.
 18. A method according to claim 11wherein said transesterification catalyst further comprises at least onealkali metal hydroxide, alkaline earth hydroxide or mixture thereof. 19.A method according to claim 1 wherein said transesterification catalystcomprises at least one alkali metal hydroxide, alkaline earth hydroxideor mixture thereof.
 20. A method according to claim 19 wherein saidalkali metal hydroxide is sodium hydroxide.
 21. A method according toclaim 1 wherein said transesterification catalyst comprises at least onealkali metal salt of a carboxylic acid, or an alkaline earth salt of acarboxylic acid, or a mixture thereof.
 22. A method according to claim21 in which said alkali metal salt of a carboxylic acid is Na₂Mg EDTA.23. A method according to claim 1 wherein said transesterificationcatalyst comprises at least one salt of comprises the salt of anon-volatile inorganic acid.
 24. A method according to claim 23 whereinsaid salt of a non-volatile acid is selected from the group consistingof NaH₂PO₃, NaH₂PO₄, Na₂H₂PO₃, KH₂PO₄, CsH₂PO₄, and Cs₂H₂PO₄.
 25. Amethod according to claim 1 wherein said mixture comprises between about0.9 and about 1.25 moles of ester-substituted diaryl carbonate per moleof aromatic dihydroxy compound present in the mixture, and between about1.0×10⁻⁸ to about 1×10⁻³ moles of transesterification catalyst per moleof aromatic dihydroxy compound present in the mixture.
 26. A methodaccording to claim 25 wherein said mixture comprises between about 0.95and about 1.05 moles of ester-substituted diaryl carbonate per mole ofaromatic dihydroxy compound present in the mixture, and between about1.0×10⁻⁶ to about 5×10⁻⁴ moles of transesterification catalyst per moleof aromatic dihydroxy compound present in the mixture.
 27. A methodaccording to claim 1 wherein said extruding is carried out on anextruder equipped with at least one vacuum vent.
 28. A method accordingto claim 27 wherein said extruder is selected from the group consistingof a co-rotating, intermeshing double screw extruder; acounter-rotating, non-intermeshing double screw extruder; a single screwreciprocating extruder, and a single screw non-reciprocating extruder.29. A method according to claim 1 wherein said mixture further comprisesa monofunctional phenol chainstopper.
 30. A method according to claim 29wherein said chainstopper is p-cumylphenol.
 31. A method for preparingpolycarbonate comprising extruding a mixture comprising bisphenol A,bis-methyl salicyl carbonate and a transesterification catalyst, at oneor more temperatures in a range between about 100° C. and about 350° C.and at one or more screw speeds in a range between about 50 rpm andabout 500 rpm, to form a product polycarbonate having a molecularweight, said mixture comprising between about 0.95 and about 1.05 molesof bis-methyl salicyl carbonate per mole of aromatic dihydroxy compoundpresent in the mixture.
 32. A method according to claim 31 wherein saidmixture comprises between about 1.0×10⁻⁸ and about 1×10⁻³ moles oftransesterification catalyst per mole of bisphenol A introduced.
 33. Amethod according to claim 32 wherein the molecular weight of the productpolycarbonate is controlled by controlling the relative amounts ofbisphenol A and bis-methyl salicyl carbonate.
 34. A method according toclaim 32 wherein said transesterification catalyst is selected from thegroup consisting of quaternary ammonium compounds, quaternaryphosphonium compounds, alkali metal and alkaline earth metal salts ofcarboxylic acids, alkali metal and alkaline earth metal hydroxides,salts of non-volatile acids; and a mixture thereof.
 35. A methodaccording to claim 34 wherein said transesterification catalystcomprises tetrabutyl phosphonium acetate.
 36. A method of preparingpolycarbonate comprising the following steps: Step (I) heating a mixturecomprising bisphenol A, bis-methyl salicyl carbonate and atransesterification catalyst at a temperature in a range between 150° C.and 200° C. and a pressure between about 1 mmHg and about 100 mmHg whileremoving by-product methyl salicylate to afford a partially crystallineprecursor polycarbonate; Step (II) grinding said partially crystallineprecursor polycarbonate; and Step (III) extruding said partiallycrystalline precursor polycarbonate at one or more temperatures in arange between about 100° C. and about 350° C., and at one or more screwspeeds in a range between about 50 and about 500 rpm.
 37. A methodaccording to claim 36 wherein said mixture comprises between about 0.95and about 1.05 moles bis-methyl salicyl carbonate per mole of bisphenolA and between about 1×10⁻⁸ and 1×10⁻³ moles of transesterificationcatalyst per mole of bisphenol A present in the mixture.
 38. A methodaccording to claim 37 wherein said mixture further comprises betweenabout 0.001 and about 0.05 mole of p-cumylphenol per mole of bisphenolA.
 39. A method according to claim 36 wherein said transesterificationcatalyst is selected from the group consisting of quaternary ammoniumcompounds, quaternary phosphonium compounds, alkali metal and alkalineearth metal salts of carboxylic acids, alkali metal and alkaline earthmetal hydroxides, salts of non-volatile acids; and a mixture thereof.40. A method of preparing polycarbonate comprising the following steps:Step (I) heating a mixture comprising bisphenol A, bis-methyl salicylcarbonate and a transesterification catalyst at a temperature in a rangebetween 150° C. and 200° C. and a pressure between about 1 mmHg andabout 100 mmHg while removing by-product methyl salicylate to afford aprecursor polycarbonate; Step (II) extruding said precursorpolycarbonate at one or more temperatures in a range between about 100°C. and about 350° C., and at one or more screw speeds in a range betweenabout 50 and about 500 rpm.
 41. A method according to claim 40 whereinsaid precursor polycarbonate is a melt.
 42. A method according to claim40 wherein said precursor polycarbonate is an amorphous solid.